A well-known negative ions generating circuit was disclosed in U.S. Pat. No. 4,872,083 entitled “method and circuit for balance control of positive and negative ions from electrical A.C. air ionizers”, which has a capacitor and a by-pass resistor. The capacitor is interposed between discharge electrode and A.C. high voltage source for generating a D.C. bias voltage between the discharge electrode and ground. The by-pass resistor is connected to the capacitor in parallel for providing a path to ground to bleed off excess bias. The by-pass resistor is an adjustable resistor whose resistance value is determined by a compensative value that is provided by the A.C. air ionizer to measure air environmental contamination, thereby to provide an equilibrium bias to generate the correct ratios of positive and negative ion currents and balance the densities of positive and negative ion. However, high frequency accompanying high voltage produced by the transformer is not mentioned how to be eliminated in the U.S. Pat. No. 4,872,083.